Multilayer, stretched heat-sealable polypropylene film

Abstract

The invention relates to sealable polypropylene films having an improved water vapor barrier effect through the following layer structure: ABCBA where a) C is the base layer of highly isotactic polypropylene with an isotacticity of >94% and a hydrocarbon resin which has a molecular weight of up to about 2000 g/mol and a softening point above 130 DEG C., b) the B's are two jacket layers of highly isotactic polypropylene (isotacticity >94%) free from hydrocarbon resin, the ratio between the thickness of the jacket layer and the thickness of the base layer being between 0.01 and 0.1, c) the A's are two surface layers of a heat-sealable polyolefin homopolymer, copolymer or terpolymer, these surface layers containing at least one antiblocking agent, a lubricant and an antistatic agent.

We claim: 
     
       1. A biaxially oriented, sealable polypropylene film having an improved water vapor barrier effect, having the following structure:   ABCBA     where   a) C is the base layer of highly isotactic polypropylene with an isotacticity of >94% and a hydrocarbon resin which has a molecular weight of up to about 2000 g/mol and a softening point above 130° C.,   b) the B's are two jacket layers of highly isotactic polypropylene (isotacticity >94%) free from hydrocarbon resin, the ratio between the thickness of the jacket layer and the thickness of the base layer being between 0.01 and 0.1,   c) the A's are two surface layers of a heat-sealable polyolefin homopolymer, copolymer or terpolymer, these surface layers containing at least one antiblocking agent, a lubricant and an antistatic agent.   
     
     
       2. A sealable film as claimed in claim 1, wherein the base layer contains a hydrocarbon resin from the group of petroleum resins, terpene resins or coal tar resins. 
     
     
       3. A sealable film as claimed in claim 2, wherein the hydrocarbon resin is a hydrogenated oligomeric cyclopentadiene resin. 
     
     
       4. A sealable film as claimed in claim 1, wherein the surface layers consist essentially of a statistical propylene/ethylene copolymer   a statistical propylene/1-butene copolymer   a statistical propylene/ethylene/olefin terpolymer   mixtures of two or three of the polymers mentioned above.   
     
     
       5. A sealable film as claimed in claim 1, wherein the surface layers contain silicon dioxide, calcium carbonate, polymethyl methacrylate, polycarbonate, silicone or HDPE (high-density polyethylene) as antiblocking agents. 
     
     
       6. A sealable film as claimed in claim 1, wherein the base layer, the surface layers, or both, contain a lubricant. 
     
     
       7. A film as claimed in claim 1, wherein the base layer, the surface layers, or both, contain an antistatic agent. 
     
     
       8. A film as claimed in claim 1, pretreated on one or both sides by corona, flame, plasma or fluorine pretreatment.